1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read/write terminal. The present invention more specifically relates to transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal. The present invention applies to such transponders, be they read only transponders, that is, adapted to operating with a terminal only reading the transponder data, or read/write transponders, which contain data that can be modified by the terminal.
2. Discussion of the Related Art
Electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write terminal side. These circuits are intended to be coupled by a close magnetic field when the transponder enters the field of the read/write terminal. The range of a transponder system, that is, the maximum distance from the terminal at which a transponder is activated (awake) depends, especially, on the size of the transponder""s antenna, on the excitation frequency of the coil of the oscillating circuit generating the magnetic field, on the intensity of this excitation, and on the transponder power consumption.
FIG. 1 very schematically shows, in a functional way, a conventional example of a data exchange system between a read/write terminal 1 (STA) and a transponder 10 (CAR).
Generally, terminal 1 is essentially formed of an oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1, between an output terminal 2p of an amplifier or antenna coupler 3 (DRIV) and a terminal 2m at a reference potential (generally, the ground). Amplifier 3 receives a high-frequency transmission signal Tx, provided by a modulator 4 (MOD). The modulator receives a reference frequency, for example, from a quartz oscillator 5 and, if necessary, a data signal to be transmitted. In the absence of a data transmission from terminal 1 to transponder 10, signal Tx is used as a power source only, to activate the transponder if said transponder enters the field. The data to be transmitted come from an electronic system, generally digital, for example, a microprocessor 6 (xcexcP).
The connection node of capacitor C1 and inductance L1 forms, in the example shown in FIG. 1, a terminal for sampling a data signal Rx, received from a transponder 10 and intended for a demodulator 7 (DEM). An output of the demodulator communicates (if necessary via a decoder (DEC) 8) the data received from transponder 10 to microprocessor 6 of read/write terminal 1. Demodulator 7 receives, generally from oscillator 5, a clock or reference signal for a phase demodulation. The demodulation may be performed from a signal sampled between capacitor C1 and resistor R1 and not across inductance L1. Microprocessor 6 communicates (bus EXT) with different input/output (keyboard, screen, means of transmission to a provider, etc.) and/or processing circuits. The circuits of the read/write terminal draw the energy necessary for their operation from a supply circuit 9 (ALIM), connected, for example, to the electric supply system.
On the side of transponder 10, an inductance L2, in parallel with a capacitor C2, forms a parallel oscillating circuit (called a reception resonant circuit) intended for capturing the field generated by series oscillating circuit L1C1 of terminal 1. The resonant circuit (L2, C2) of transponder 10 is tuned on the frequency of the oscillating circuit (L1C1) of terminal 1.
Terminals 11, 12, of resonant circuit L2C2, which correspond to the terminals of capacitor C2, are connected to two A.C. input terminals of a rectifying bridge 13 formed, for example, of four diodes D1, D2, D3, D4. In the representation of FIG. 1, the anode of diode D1 and the cathode of diode D3 are connected to terminal 11. The anode of diode D2 and the cathode of diode D4 are connected to terminal 12. The cathodes of diodes D1 and D2 form a positive rectified output terminal 14. The anodes of diodes D3 and D4 form a reference terminal 15 of the rectified voltage. A capacitor Ca is connected to rectified output terminals 14, 15 of bridge 13 to store power and smooth the rectified voltage provided by the bridge. It should be noted that the diode bridge may be replaced with a single-halfwave rectifying assembly.
When transponder 10 is in the field of terminal 1, a high frequency voltage is generated across resonant circuit L2C2. This voltage, rectified by bridge 13 and smoothed by capacitor Ca, provides a supply voltage to electronic circuits of the transponder via a voltage regulator 16 (REG). These circuits generally include, essentially, a microprocessor (xcexcP) 17 (associated with a memory not shown), a demodulator 18 (DEM) of the signals that may be received from terminal 1, and a modulator 19 (MOD) to transmit information to terminal 1. The transponder is generally synchronized by means of a clock (CLK) extracted, by a block 20, from the high-frequency signal recovered across capacitor C2 before rectification. Most often, all the electronic circuits of transponder 10 are integrated in a same chip.
To transmit the data from transponder 10 to unit 1, modulator 19 controls a stage of modulation (back modulation) of resonant circuit L2C2. This modulation stage is generally formed of an electronic switch (for example, a transistor T) and of a resistor R, in series between terminals 14 and 15. Transistor T is controlled at a so-called sub-carrier frequency (for example, 847.5 kHz), much smaller (generally with a ratio of at least 10) than the frequency of the excitation signal of the oscillating circuit of terminal 1 (for example, 13.56 MHz). When switch T is closed, the oscillating circuit of the transponder is submitted to an additional damping as compared to the load formed of circuits 16, 17, 18, 19 and 20, so that the transponder draws a greater amount of power from the high frequency field. On the side of terminal 1, amplifier 3 maintains the amplitude of the high-frequency excitation signal constant. Accordingly, the power variation of the transponder translates as an amplitude and phase variation of the current in antenna L1. This variation is detected by demodulator 7 of terminal 1, which is either a phase demodulator or an amplitude demodulator. For example, in the case of a phase demodulation, the demodulator detects, in the half-periods of the sub-carrier where switch T of the transponder is closed, a slight phase shift (a few degrees, or even less than one degree) of the carrier of signal Rx with respect to the reference signal. The output of demodulator 7 (generally the output of a band-pass filter centered on the sub-carrier frequency) then provides an image signal of the control signal of switch T that can be decoded (by decoder 8 or directly by microprocessor 6) to restore the binary data.
FIGS. 2A and 2B illustrate a conventional example of data transmission from terminal 1 to a transponder 10. FIG. 2A shows an example of shape of the excitation signal of antenna L1 for a transmission of a code 1011. The modulation currently used is an amplitude modulation with a 106-kbit/s rate (one bit is transmitted in approximately 9.5 xcexcs) much smaller than the frequency (for example, 13.56 MHz) of the carrier coming from oscillator 5 (period of approximately 74 ns). The amplitude modulation is performed either in all or nothing or with a modulation ratio (defined as being the difference of the peak amplitudes between the two states (0 and 1), divided by the sum of these amplitudes) smaller than one hundred percent (100%) due to the need for supply of transponder 10.
In the example of FIGS. 2A and 2B, the carrier at 13.56 MHz is modulated in amplitude, with a 106-kbit/s rate, with a modulation ratio of, for example, 10%. Then, as will be better understood by comparing the time scales of FIGS. 2A and 2B, the transmission of a bit from terminal 1 to transponder 10 requires, with such a rate, sixty-four halfwaves of the carrier. FIG. 2B shows eight halfwaves of the carrier at 13.56 MHz.
FIGS. 3A and 3B illustrate a conventional example of a data transmission from transponder 10 to terminal 1. FIG. 3A illustrates an example of the shape of the control signal of transistor T, provided by modulator 19, for a transmission of a code 0110. On the transponder side, the back modulation is generally of resistive type with a carrier (called a sub-carrier) of, for example, 847.5 kHz (period of approximately 1.18 xcexcs). The back modulation is, for example, based on a BPSK-type (binary phase-shift keying) coding at a rate on the order of 106 kbits/s, much smaller than the sub-carrier frequency.
FIG. 3B illustrates the variations of the sub-carrier during a time interval of same length as in FIG. 2B (eight halfwaves of the carrier at 13.56 MHz), that is, of substantially 1.18 xcexcs, corresponding to one sub-carrier period.
It should be noted that, whatever the type of modulation or back modulation used (for example, amplitude, phase, frequency) and whatever the type of data coding (NRZ, NRZI, Manchester, ASK, BPSK, etc.), this modulation or back modulation is performed digitally, by jumping between two binary levels.
It should also be noted that the terminal does not transmit data when it receives some from a transponder, the data transmission occurring alternately in one direction, then in the other (half-duplex).
Indeed, the previously-described conventional transmission method does not enable simultaneous transmission from the transponder to the terminal and from the terminal to the transponder. Among the problems raised by such a bidirectional method, there is, in particular, the risk for the transmission from one of the system elementsxe2x80x94terminal or transponderxe2x80x94to the otherxe2x80x94transponder or terminalxe2x80x94to disturb the decoding of the data received from the other element.
The present invention aims at providing a novel transmission method that is bidirectional (duplex method).
The present invention also aims at providing a read/write terminal useable to implement a conventional method (half-duplex) as well as a bidirectional method (duplex).
The present invention also aims at providing a transponder adapted to implementing a conventional transmission method as well as a bidirectional method.
To achieve these and other objects, the present invention provides a method of transmission between two elements chosen from a terminal and a transponder, each element including an oscillating circuit, a modulation means and a demodulation means, including the steps of simultaneously performing a transmission in amplitude modulation of a signal transmitted from a first to a second element and a transmission of a signal from the second to the first element adapted to being submitted to a phase demodulation in the latter, and wherein the amplitude modulation ratio is smaller than 100%.
According to an embodiment of the present invention, the modulation ratio is smaller than 50%.
According to an embodiment of the present invention, the method includes, on the side of the first element, clipping the received signal before demodulation.
According to an embodiment of the present invention, the method includes, on the side of the first element, clipping a reference signal of the phase demodulation.
According to an embodiment of the present invention, the first element is a transponder, the second element being a terminal.
According to an embodiment of the present invention, the first element is a terminal, the second element being a transponder.
The present invention also provides a terminal including a means for comparing transmitted and received signals clipped by respective clipping means.
According to an embodiment of the present invention, the terminal includes means for regulating the signal phase in its oscillating circuit with respect to a reference value, the response time of the phase regulation being shorter than the amplitude modulation period and longer than the period of the signal that will have to undergo a phase demodulation.
The present invention also provides a transponder including an amplitude demodulation means sized according to the attenuation introduced by a back modulation means.
According to an embodiment of the present invention, the transponder includes an amplitude demodulation means, the result of which is only taken into account during one state out of two of an output signal of a back modulation means.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.